1. Field of the Invention
The present invention relates to a data transfer method and a data transfer apparatus for transferring data in a parallel system through communication buses between an information processing apparatus and at least one other information processing apparatus, and further relates to an information processing apparatus provided with such a data transfer apparatus.
2. Description of Background Art
In parallel data transfer through a communication bus, there is a limit in the quantity of data, or the number of bytes of data, which can be transferred in one operation. Accordingly, in the case where data having a significant length exceeding the limit (hereinafter referred to as "significant length data"), it is impossible to transfer the data in one operation. Conventionally, therefore, the significant length data is divided into a plurality of blocks of data in accordance with the capacity of the communication bus so that the data is transferred block by block as disclosed, for example, in Japanese Patent Unexamined Publication JP-A-62-217340.
According to the conventional technique disclosed in the above Japanese Patent Unexamined Publication No. 62-217340, it is necessary to judge whether data transfer has been completed correctly or not. For example, in the case where significant length data is read out from data storage means, it is necessary that data reading be made twice with a fixed period of access to the data storage means. The data read out twice are collated whether they are coincident with each other or not to thereby check whether correct data transfer has been performed or not.
Accordingly, the conventional technique disclosed in the above Japanese Patent Unexamined Publication No. 62-217340 has a problem that it is impossible to make access to the data storage means at random or non-synchronously. If access to the data storage means is made at random or non-synchronously without performing data collating, there is a possibility that significant length data having erroneous contents may be transferred.
The conventional technique has a further problem that there is a limit in making the data transfer rate high, because it is necessary to carry out reading operation twice for the purpose of collating.
In the case where data is transferred between computer systems, conventionally, simultaneously generated access requests to data storage means are controlled by a flag. In this case, however, there is a possibility of occurrence of an erroneous operation that both one and another access request may be allowed if the other access request is generated in addition to the one current access request in a period from the point of time of confirmation of the not-set state of the flag to the point of time of setting the flag according to the one current access request.
Further, time is required to perform flag control processing so that there is a limit in making the data transfer rate high.